The blood contains the elements for building up, supplying the waste of,
and nourishing the whole animal economy. On making an examination of the
blood with a microscope, it is found full of little red globules, which
vary in their size and shape in different animals, and are more numerous
in the warm than in the cold-blooded. Probably this arises from the fact
that the latter absorb less oxygen than the former. When blood stands
for a time after being drawn, it separates into two parts. One is called
_serum_, and resembles the white of an egg; the other is the clot, or
crassamentum, and forms the red coagulum, or jelly-like substance. This
is accompanied by whitish tough threads, called _fibrine_.
[Illustration: THE HEART VIEWED EXTERNALLY.
_a_, the left ventricle; _b_, the right ventricle; _c_, _e_, _f_, the
aorta; _g_, _h_, _i_, the carotid and other arteries springing from the
aorta; _k_, the pulmonary artery; _l_, branches of the pulmonary artery
in the lungs; _m_, _m_, the pulmonary veins emptying into the left
auricle; _n_, the right auricle; _o_, the ascending vena cava; _q_, the
descending vena cava; _r_, the left auricle; _s_, the coronary vein and
artery. (See _Circulation of the Blood_, on the opposite page.)]
When blood has been drawn from an animal, and it assumes a cupped or
hollow form, if serum, or buffy coat, remains on its surface, it denotes
an impoverished state; but if the whole, when coagulated, be of one
uniform mass, it indicates a healthy state of that fluid. The blood of a
young animal, provided it be in health, coagulates into a firm mass,
while that of an old or debilitated one is generally less dense, and
more easily separated. The power that propels the blood through the
different blood-vessels is a mechanico-vital power, and is accomplished
through the involuntary contractions and relaxations of the heart; from
certain parts of which arteries arise, in other parts veins terminate.
(See Plate.)
The heart is invested with a strong membranous sac, called
_pericardium_, which adheres to the tendinous centre of the diaphragm,
and to the great vessels at its superior portion. The heart is
lubricated by a serous fluid, secreted within the pericardium, for the
purpose of guarding against friction. When an excess of fluid
accumulates within the sac, it is termed dropsy of the heart. The heart
is divided into four cavities, viz., two auricles, named from their
resemblance to an ear, and two ventricles, (as seen at _a_, _b_,)
forming the body. The left ventricle is smaller than the right, yet its
walls are much thicker and stronger than those of the latter: it is from
this part that the large trunk of the arteries proceed, called the
_great aorta_. The right cavity, or ventricle, is the receptacle for
blood returned by the venous structure after having gone the rounds of
the circulation; the veins terminating, as they approach the heart, in a
single vessel, called _vena cava_, (see plate, _o_, _q_, ascending and
descending portion.) The auricle on the left side of the heart receives
the blood that has been distributed through the lungs for purification.
Where the veins terminate in auricles, there are valves placed, to
prevent the blood from returning. For example, the blood proceeds out of
the heart along the aorta; the valve opens upwards; the blood also
moves upwards, and raises the valve, and passes through; the pressure
from above effectually closes the passage. The valves of the heart are
composed of elastic cartilage, which admits of free motion. They
sometimes, however, become ossified. The heart and its appendages are,
like other parts of the system, subject to various diseases, which are
frequently very little understood, yet often fatal. Now, the blood,
having passed through the veins and vena cava, flows into the right
auricle; and this, when distended, contracts, and forces its contents
into the right ventricle, which, contracting in its turn, propels the
blood into the pulmonary arteries, whose numerous ramifications bring it
in contact with the air-cells of the lungs. It then, being deprived of
its carbon, assumes a crimson color. Having passed through its proper
vessels, it accumulates in the left auricle. This also contracts, and
forces the blood through a valve into the left ventricle. This ventricle
then contracts in its turn, and the blood passes through another valve
into the great aorta, to go the round of the circulation and return in
the manner just described.
Many interesting experiments have been made to estimate the quantity of
blood in an animal. "The weight of a dog," says Mr. Percival, "being
ascertained to be seventy-nine pounds, a puncture was made with the
lancet into the jugular vein, from which the blood was collected. The
vein having ceased to bleed, the carotid artery of the same side was
divided, but no blood came from it; in a few seconds afterwards, the
animal was dead. The weight of the carcass was now found to be
seventy-three and a half pounds; consequently it had sustained a loss of
five and a half pounds--precisely the measure of the blood drawn. It
appears from this experiment, that an animal will lose about one
fifteenth part of its weight of blood before it dies; though a less
quantity may so far debilitate the vital powers, as to be, though less
suddenly, equally fatal. In the human subject, the quantity of blood has
been computed at about one eighth part of the weight of the body; and as
such an opinion has been broached from the results of experiments on
quadrupeds, we may fairly take that to be about the proportion of it in
the horse; so that if we estimate the weight of a horse to be thirteen
hundred and forty-four pounds, the whole quantity of blood will amount
to eighty-four quarts, or one hundred and sixty-eight pounds; of which
about forty-five quarts, or ninety pounds, will commonly flow from the
jugular vein prior to death; though the loss of a much less quantity
will deprive the animal of life."